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Spring freeze injury occurs whenever low temperatures coincide with sensitive plant growth stages. Injury can cover large areas or only a few fields or parts of fields. It is more severe along river bottoms, valleys and depressions in fields where cold air settles.
The degree of injury to wheat from spring freezes is influenced by the low temperature, the duration of low temperature, and the plant growth stage. Prolonged exposure to freezing causes much more injury than brief exposure to the same temperature. Temperatures at which injury can be expected are shown in Table 1, and are for two hours of exposure to each temperature. Less injury can be expected from shorter exposure times, while injury might be expected at even somewhat high temperatures from longer exposure.
The many factors influencing freeze injury to wheat -- plant growth stage, plant moisture content, and duration of exposure -- make it difficult to predict the extent of injury. This is complicated further by differences in elevation and topography among wheat fields and between the fields and official weather stations. It is not unusual, for instance, for wheat growers to report markedly lower temperatures than are recorded at the nearest official weather station.
During the jointing stage, when heads are moving up the stems, it takes about two hours at 24oF to cause moderate to severe injury to wheat. Injury symptoms at this stage include: death of the growing point; leaf yellowing or burning; lesions, splitting, or bending of the lower stem; and silage odor. Stem growth stops immediately when the growing points are injured, but growth from later tillers may obscure damage. Partial injury at this stage may cause a mixture of normal tillers and late tillers and result in uneven maturity and some decrease in grain yield.
With wheat in the joint to boot stage wait several days to determine injury. With wheat that is heading to before flowering, the first visible freeze-damage symptoms occur to the anthers. They become shriveled and twisted while retaining their lime green color. This damage can be detected with a hand-held lens within 24 hours after a freeze.
Early-maturing wheat is more likely to be injured by freezes than late-maturing wheat. Susceptibility to freezing temperatures steadily increases as maturity of wheat advances during spring. Some varietal difference in resistance to spring freeze injury has been reported, but it is mostly caused by differences in plant growth stages at the time of the freeze. There is little difference among wheat varieties at the same growth stage and, therefore, little opportunity to increase freezing resistance in improved varieties.
When growing conditions are favorable and there is high soil fertility, particularly nitrogen enriched soil, wheat is more sensitive to freeze injury because of its lush growth and high moisture content. Conversely, drought stress tends to harden plants to cold and decreases their water content, thus reducing the severity of freeze injury. However, ample soil moisture, cool temperatures, and high soil fertility slow plant maturity, so injury is sometimes less severe than in plants that have had less favorable growing conditions and are at a more advanced growth stages when freezing occurs.
Wheat is most sensitive to freeze injury during reproductive growth, which begins with pollination during late boot or heading stages. Temperatures that are only slightly below freezing can severely injury wheat at these stages and greatly reduce grain yields.
Fortunately, last week’s cold temperatures were accompanied by rain and snow, which helped moderate temperatures within the wheat canopies. Since it is difficult to know what the air temperatures were in the wheat canopy, it is often necessary to wait five days or so after the freeze event and then assess plant damage. The growing points can be located by splitting stems lengthwise with a sharp knife. A normal, uninjured growing point is bright yellow-green and turgid; freeze injury causes it to become white or brown and water-soaked in appearance. It takes several days of warm weather for freeze injury to become apparent. Temperatures in a dense wheat canopy with moist soils are likely to cool off more slowly than surrounding air temperatures. Damage within a dense wheat canopy is likely to be less than in a thin stand of wheat.
For more information on how to assess freeze injury in winter wheat, see Freeze Injury to Nebraska Wheat (EC94-132), which can also be found by visiting the Wheat Production Systems Web site at wheatbook.unl.edu.
Drew Lyon
Extension Dryland Crops Specialist
Panhandle REC
Robert Klein
Cropping Systems Specialist
West Central REC
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The results of our updated comparison between continuous corn and corn-soybean are very similar to our report in March where the corn-soybean rotation had an advantage of $79.60 per acre. Our revised analysis reports a difference of $63 per acre with most of that difference resulting from using a lower soybean price and a higher yield for continuous corn. In spite of the changes in the production costs budgeted, the difference in costs between the continuous corn and corn-soybean rotations was within $1 of our earlier estimate.
To compare corn-corn-soybean to corn-soybean we used our yields from continuous corn for the second year of corn in the corn-corn-soybean rotation. Our analysis shows the corn-soybean rotation with an advantage of $21 per acre (the difference in net remaining for land, management, machinery interest and overheads after covering our budgeted costs). At $5.50 per bushel it would require 3.8 bushels of soybeans to make up the $21 difference and since only one-third of the acreage is in soybeans in a corn-corn-soybean rotation, an 11.4 bushel per acre soybean yield increase would be required for a breakeven (3.8 x 3 = 11.4). That would be a yield increase of over 20% using our 10-year average soybean yield of 56 bushels per acre.
Another comparison that is likely of interest given the soybean rust threat is how much of a soybean yield loss would be required to favor planting more corn? Our budgets indicate a 40% soybean yield reduction or a 33 bushel per acre soybean yield would result in a breakeven between the continuous corn, corn-soybean and corn-corn-soybean rotations.
Roger Selley
Extension Agricultural Economist
South Central Ag Lab
Bob Klein
Extension Crops Specialist
West Central REC
Terry Hejny
Extension Educator, Fillmore County
How chloride can influence soybean yield response to irrigation
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Once in the leaves, the plant experiences symptoms of water stress even though adequate soil water is present. Not all soybean varieties exhibit this trait; however, preliminary trial results suggest that varieties planted in Nebraska could exhibit this trait. In fact, of the seven varieties evaluated using the Arkansas research protocol, six exhibited the trait of pumping chloride to the leaves.
How could this become a Nebraska issue? Chloride dissolves in water and will move through the soil profile similar to nitrates. Arkansas has areas where the natural chloride concentration can exceed 200 ppm. In Arkansas they have also documented 20-30% yield reductions due to irrigating with groundwater high in chloride.
Though Nebraska has few, if any, areas with natural concentrations at that level, we do have the potential for man-made areas. Some water sample results acquired from a commercial laboratory seem to suggest that areas with long-term animal production facilities could have elevated chloride levels. The level of salt in the animal’s normal diet, the manure distribution history for the area, and the length of time the feedlot has been in operation may contribute to this issue.
Another concern would be for areas with long-term histories of potash application. Since potash is potassium chloride, applications to meet plant potassium needs could result in chloride leaching. Certainly areas with both factors would have a greater potential for elevated chloride levels.
What is unclear is the concentration required to limit yield response to irrigation and which varieties would be affected. For example, yield response to irrigation might become limited when irrigating with water at a chloride concentration of 30-50 ppm or lower. This scenario would result in dryland yields being similar to irrigated yields. One indicator of whether chloride could be an issue is groundwater nitrate concentration. An Extension project in Holt County found that groundwater nitrate and chloride concentrations tracked each other over a three- to four-year period. That particular site was irrigated and had a history of potash applications on a loamy sand soil, with shallow groundwater.
If your field site has received multiple applications of a material containing chloride, and soybean don’t seem to respond to irrigation, it might be beneficial to have the irrigation water supply analyzed for chloride content. This test generally costs less than $10 and could provide one factor that limits soybean yield.
Since not all soybean varieties exhibit this trait, one option for those with elevated chloride concentrations is to request information about the varieties being planted. In the short-term, a limited amount of information likely will be available on most Nebraska hybrids. As time goes on, more information should be available. The other option is to plant some other crop on the acres affected by elevated chloride levels.
Bill Kranz
Extension Irrigation Specialist
Charles Shapiro
Extension Soils Specialist,
both at the Northeast REC
Charles Wortmann
Extension Nutrient Management Specialist
Rusts developing south of Nebraska
Rains may lead to increasing wheat diseases
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Frequent rains have been ideal for the development of tan spot in fields with wheat stubble from the previous wheat crop. Tan spot is recognized by the presence of dark brown spots with tan centers on leaves. So far the spotting is primarily on the lower leaves, but with continued wet weather. the infection will progress upward onto the upper leaves.
Overcast weather has promoted the development of powdery mildew on the lower half of wheat plants. If the current weather pattern continues, it to will move higher on the plants.
So far we have not had any wheat samples sent to the UNL Plant Disease Clinic with either leaf rust or stripe rust; however, both diseases are very active in Oklahoma and moderately active in southern Kansas. Oklahoma reports leaf rust severities of up to 60% on the upper leaves of Jagalene and Jagger.
Jagalene is the most popular variety in Nebraska Although, previously reported as resistant, the development of moderately high severities with a susceptible reaction on Jagalene could signal that that resistance is breaking down due to new races of the leaf rust pathogen. Growers may want to monitor their Jagalene fields regularly for leaf rust development.
Stripe rust is widely spread across Oklahoma including heavy rusting in the Oklahoma Panhandle. It is present in both irrigated and dryland wheat. Its presence in western Oklahoma could signal potential stripe rust problems for southwest Nebraska and the Panhandle. Continued dry weather in these areas will restrict stripe rust development on dryland wheat. Wheat growers in the rest of Nebraska should monitor their fields for both leaf rust and stripe rust development.
John Watkins
Extension Plant Pathologist
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While the rains helped yields, when combined with a long season, they allowed for some late season weeds to grow as the residual herbicides ran out. This was evident primarily in grain sorghum production where there aren’t as many herbicide choices and crop growth was slower because of the lack of heat. The no-till treatment in this study had grass weed control problems on both sets of plots, primarily with sandburs creeping in from the field margins over the last few years.
Each year tires carry the burs further into the field, indicating the need for weed control, even in field margins. Sandburs were not a major problem in the first 20 years of these plots and have started increasing recently. Rotation with Roundup Ready soybeans has helped keep them in check, but not enough to control them. Actually, overspray onto the field margins has killed some of the desirable bromegrass and has allowed the later season sandburs to grow without competition. In the plots, row crop cultivation has provided enough additional control to reduce the spread in those treatments. With replicated treatments, the variation in yield reduction can be seen when looking at individual plots. The no-till replication on the field edge, next to a grassy access road with sandburs, yielded only 114.9 bu/ac. The no-till replication on the other end of the plots, next to end rows where sandburs are creeping in, yielded 128.7 bu/ac. With very few sandburs, the replication in the center of the plot area yielded 146.1 bu/ac. Likewise, the no-till with cultivation replications yielded 146.2, 156.3, and 142.6 bu/ac, respectively, showing the value of row crop cultivation when weed control is needed.
As there are no effective herbicide treatments for sandbur control in grain sorghum, a management change is needed to address this problem in continuous no-till. This season, year 25 of the study, one set of the plots will be switched to a corn/soybean rotation and Roundup Ready corn will be used to allow a wider selection of herbicides for sandbur control. The other set of plots, in soybeans this year, will continue in the long-term grain sorghum/soybean rotation.
As with any production system, small problems need to be addressed so that they don’t expand into large problems over the years. Crop rotation, which allows herbicide rotation, is a valuable tool to address many pest problems, especially in continuous no-till. The addition of corn to this study will provide new data on corn in established long-term tillage systems.
Paul Jasa
Extension Engineer
Pre-emergence herbicides
Soil-applied herbicides available for use in sunflower are EPTC (Eptam®), ethalfluralin (Sonalan®), S-metolachlor (Dual Magnum®), pendimethalin (Prowl® H2O and others), sulfentrazone (Spartan®), and trifluralin (Treflan™ and others). Eptam, Sonalan, and Treflan require mechanical incorporation soon after application. Prowl requires incorporation with tillage or rainfall within seven days of application or performance will suffer. Although there are many benefits to growing sunflower in no-till or reduced-tillage systems, cheap, effective, and consistent weed control is most easily achieved with preplant incorporated herbicide treatments.
With the exception of Spartan, soil-applied herbicides primarily control grassy weeds such as foxtail, sandbur, stinkgrass, and witchgrass. All these grass control herbicides provide some level of control of small-seeded broadleaf weeds such as pigweed and lambsquarters. At higher use rates, they also can provide acceptable to poor control of kochia and Russian thistle. Spartan provides control of pigweed, kochia, Russian thistle, and lambsquarters. Spartan should be tank-mixed with a herbicide that will control grasses to obtain optimum weed control. Some crop injury has been reported from using Spartan. The injury is typically restricted to high pH, low organic matter soils and consists of leaf chlorosis, plant stunting, and occasionally plant death. Injured plants generally grow out of the injury within a few weeks and yield differences are minimal at harvest.
Postemergence herbicides
Imazamox (Beyond™) is labeled for use only in Clearfield sunflower hybrids. Clearfield sunflower hybrids contain a gene that confers tolerance, not resistance, to Beyond herbicide. Some slight crop injury (leaf yellowing and plant stunting) may be observed following Beyond herbicide application to Clearfield sunflower hybrids, especially where overapplication occurs such as in spray overlaps or field ends. Injured plants often recover quickly. Non-Clearfield hybrids would be killed if treated with Beyond herbicide.
Beyond herbicide should be applied early postemergence at a rate of 4 oz of product per acre to sunflower with two to eight leaves. Weeds should be actively growing at the time of application and broadleaf weeds should be less than 3 inches tall. Grass weeds should have no more than four to five leaves. A nonionic surfactant and nitrogen based fertilizer must be added to the spray solution for optimum weed control. It is recommended that a soil-applied grass herbicide, such as Prowl® or Dual Magnum®, be applied prior to application of Beyond. Beyond will control many broadleaf weeds that are troublesome in Nebraska sunflower fields including pigweed, kochia, Russian thistle, and nightshade. Beyond inhibits ALS-AHAS synthesis in weeds and will not effectively control ALS-resistant kochia or Russian thistle. There is also some concern about the risk of transferring the gene conferring tolerance to Beyond herbicide from commercial sunflower hybrids to wild sunflower. This technology should be avoided in fields with wild sunflower.
See the Table of Postemergence Herbicides for Sunflower for details on the rates for various soil types and approximate costs. The other postemergence herbicides used in sunflower are sethoxydim (Poast) and clethodim (Select). Both of these herbicides control emerged grass weeds and volunteer small grains.
For additional information on weed management in Nebraska crops, see the 2005 Guide for Weed Management in Nebraska (EC05-130), published by University of Nebraska Cooperative Extension. It is available online and at local Cooperative Extension offices.
Drew LyonHerbicide options for sunflower
Weed control in sunflower begins with good weed control in the previous crop and placing sunflower in a rotation where warm-season broadleaf weeds are easily controlled in previous crops. Sunflower is usually planted at lower densities than many other crops and it grows slowly during the first two to three weeks. Weeds that emerge and establish during this time can be very competitive and reduce sunflower yield potential tremendously; however, sunflower is a strong competitor with weeds that emerge three or more weeks after sunflower emergence. Therefore, maintaining weed-free sunflower for the first three to four weeks after planting will minimize yield losses from weeds.
Extension Dryland Cropping Systems Specialist
Robert Wilson
Extension Weeds Specialist
Both at the Panhandle REC
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While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists. While things will be getting very busy in the next few weeks as row crops are planted, if you’re growing high quality alfalfa hay take the time to monitor fields for weevils over the next month.
Clover leaf weevils (CLW) are occasionally a problem but are very vulnerable to fungus disease and so haven’t been pests since the late 80s early 90s when spring rains were rare. Dry conditions over the past several years may have helped increase populations, especially in western Nebraska, although last week’s rain may have knocked populations down. Clover leaf weevil larvae will be in the debris around the crowns during day. Scratching in the soil around the crowns and counting the number of larvae found per crown will help give a better idea of clover leaf weevil infestation. Their brown heads will help distinguish them from the black-headed alfalfa weevil. The chart compares characteristics of the alfalfa weevil and clover leaf weevil.
Both the alfalfa weevil and clover leaf weevil feed on first cutting alfalfa as larvae and regrowth of the first cutting as adults. While research in northeast Nebraska has shown that clover leaf weevil larva feeding does not cause yield reduction to first cutting alfalfa, alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting.
It is essential that fields be monitored for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are small (1/16 to 3/8 inch in length) and pale yellowish green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back. The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed.
Once the alfalfa is about 4-6 inches tall, take a bucket, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of the bucket. Average the number of weevil larvae per stem. Use the economic threshold charts (Figure 2) to help determine whether to treat. Each chart has been developed for a different alfalfa value. To treat or re-sample depends on the average number of weevils per stem, the stem length, and the value of the alfalfa. When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than treat it. Insecticides registered to control alfalfa weevil larvae include Ambush, Baythroid, Cythion, Furadan, Guthion, Imidan, Lannate, Lorsban, Mustang Max, Penncap M, Pounce, Proaxis, Sevin, and Warrior. Check the labels or visit the UNL Department of Entomology Web site to find the use rates.
Keith Jarvi
Extension IPM Assistant
Northeast REC, Norfolk
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Bean breeder joins faculty at Panhandle REC in Scottsbluff
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Urrea comes to the Panhandle from Nepal, where he spent nearly two years leading a project to develop high-quality varieties of maize that grow on hillsides to help alleviate malnutrition in that country. Prior to Nepal, he worked as a research associate scientist at the International Center for Maize and Wheat Improvement (CIMMYT) in Mexico, where he developed new subtropical varieties and hybrids of corn.
He received his doctorate in plant breeding and genetics, with a minor in statistics, from North Dakota State University. At NDSU he worked as a graduate research fellow in the six-row barley breeding project. While pursuing his master’s degree at the University of Puerto Rico, he oversaw greenhouse and field operations of the entire dry bean breeding and genetics project for six months. Prior to that he was a research assistant in the breeding and genetics program at the International Center for Tropical Agriculture at Cali, Colombia. His work resulted in the release of six small and medium-seeded common dry bean cultivars possessing multiple-disease resistance in Argentina, Brazil, Bolivia and Mexico. He received a bachelor of science degree from Universidad Nacional de Colombia.
Urrea said his work in the Panhandle will include traditional breeding of dry bean varieties along with the use of genetic markers to select plants that are resistant to diseases that often reduce yields or quality, such as rust, white molds, and common bacteria blights. He said he expects much of his work will involve Great Northern beans, as well as pintos and other types of dry edible beans.
In addition to disease, Urrea said he hopes the Center’s breeding program will address other grower issues, such as water efficiency and minimum tillage systems. He also hopes to develop varieties that have a more upright architecture, allowing direct harvesting by combines rather than needing to be cut and windrowed first.
Urrea’s dry bean breeding program will build on the strong base created by Dr. Dermot Coyne, renowned UNL dry bean breeder. Coyne’s germplasm collection is being catalogued and will be moved to Scottsbluff in June 2005.
Hibberd said Urrea’s appointment is one of several developments intended to enhance the Panhandle REC’s capabilities in the dry edible bean breeding area. Other plans include construction of a new greenhouse and a molecular laboratory, which will allow molecular marker work to enhance and speed up breeding.
Cropland, pasture and rangeland are eligible. More than 1600 people have already attended informational meetings in southeast Nebraska to learn about the program, payment levels and eligibility requirements. Two final CSP public information meetings will be held next week at 7 p.m. at the following sites:
This may be the one chance in eight years for producers in the eligible watersheds to apply for the program. Enrollment is limited to producers who have most of their farming operation in one of the eligible watersheds. It is expected that the program will be offered in other Nebraska watersheds later. All or parts of Adams, Butler, Clay, Dundy, Fillmore, Gage, Hall, Hamilton, Jefferson, Kimball, Lancaster, Pawnee, Polk, Saline, Seward, Thayer and York counties are eligible for CSP enrollment this year. Dundy and Kimball counties are holding separate meetings.
Farmers may enroll their entire ag operation, if it qualifies, or as little as one qualifying field or pasture. Once a producer has a contract, they may bring their other land into the contract as that land meets minimum qualifications.
"NRCS has a short pre-screening questionnaire to help farmers determine if they meet minimum
requirements on at least one field in their operation," said Steve Chick, state conservationist with
the U.S. Department of Agriculture's Natural Resources Conservation Service. "If so, they should
complete the full self-assessment workbook and attend one of the workbook sessions, if needed."
University of Nebraska Cooperative Extension is conducting workshops to help farmers complete the workbook. Upcoming CSP Self-Assessment Workshops are slated for
Class size is limited and registrations are required for the workbook sessions. Contact the Cooperative Extension office in the host county to register.
For more information on this program and upcoming meetings, check out the Cooperative Extension Web site supporting the Conservation Security Program or the NRCS Web site.
Thomas FrantiConservation Security Program sign-up until May 27
Parts of 15 counties in southeast Nebraska and two counties in western Nebraska are eligible for the 2005 Conservation Security Program sign up. This voluntary program financially rewards farmers who meet high standards of conservation and environmental management in their farming operations.
Extension Surface Water Management Specialist
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